Travis: Test go1.9 and go1.10, too

Add some comments regarding old go versions.

.travis.yml

@@ -1,9 +1,16 @@
 sudo: false
 language: go
+# * github.com/grpc/grpc-go still supports go1.6
+#   - When we drop support for go1.6 we can remove golang.org/x/net/context
+#     below as it is part of the Go std library since go1.7
+# * github.com/prometheus/client_golang already requires at least go1.7 since
+#   September 2017
 go:
   - 1.6.x
   - 1.7.x
   - 1.8.x
+  - 1.9.x
+  - 1.10.x
   - master
 
 install: